C#编程（10_串联多个字符串）

串联是将一个字符串追加到另一个字符串末尾的过程。使用 + 运算符串联字符串文本或字符串常量时，编译器会创建一个字符串。串联不在运行时发生。但字符串变量只能在运行时串联，对此，您应该了解各种方法的性能含义。

下面的示例演示如何将一个长字符串拆分为几个较短的字符串，从而提高源代码的可读性。这些较短的字符串将在编译时串联成一个字符串。无论涉及到多少个字符串，都不会有运行时性能开销。

static void Main()
{
    // Concatenation of literals is performed at compile time, not run time.
    string text = "Historically, the world of data and the world of objects " +
    "have not been well integrated. Programmers work in C# or Visual Basic " +
    "and also in SQL or XQuery. On the one side are concepts such as classes, " +
    "objects, fields, inheritance, and .NET Framework APIs. On the other side " +
    "are tables, columns, rows, nodes, and separate languages for dealing with " +
    "them. Data types often require translation between the two worlds; there are " +
    "different standard functions. Because the object world has no notion of query, a " +
    "query can only be represented as a string without compile-time type checking or " +
    "IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to " +
    "objects in memory is often tedious and error-prone.";

    Console.WriteLine(text);
}

若要串联字符串变量，可以使用 + 或 += 运算符，也可以使用 String.Concat、String.Format 或 StringBuilder.Append 方法。+  运算符容易使用，且有利于提高代码的直观性。  即使在一条语句中使用多个 + 运算符，字符串内容也将只复制一次。但是，如果重复此操作多次（如使用循环），则可能会导致出现效率问题。例如，考虑下面的代码：

static void Main(string[] args)
{
    // To run this program, provide a command line string.
    // In Visual Studio, see Project > Properties > Debug.
    string userName = args[0];
    string date = DateTime.Today.ToShortDateString();

    // Use the + and += operators for one-time concatenations.
    string str = "Hello " + userName + ". Today is " + date + ".";
    System.Console.WriteLine(str);

    str += " How are you today?";
    System.Console.WriteLine(str);

    // Keep the console window open in debug mode.
    Console.WriteLine("Press any key to exit.");
    Console.ReadKey();
}
// Example output:
//  Hello Alexander. Today is 1/22/2008.
//  Hello Alexander. Today is 1/22/2008. How are you today?
//  Press any key to exit.
//

如果您串联的字符串数量不那么巨大（例如，在循环中），那么这些代码的性能成本可能不会很高。上述情况同样适用于 String.Concat 和 String.Format 方法。

但如果性能的优劣很重要，则应该总是使用 StringBuilder 类来串联字符串。下面的代码使用 StringBuilder 类的 Append 方法来串联字符串，因此不会有 + 运算符的链接作用产生。

class StringBuilderTest
{
    static void Main()
    {
        string text = null;

        // Use StringBuilder for concatenation in tight loops.
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        for (int i = 0; i < 100; i++)
        {
            sb.AppendLine(i.ToString());
        }
        System.Console.WriteLine(sb.ToString());

        // Keep the console window open in debug mode.
        System.Console.WriteLine("Press any key to exit.");
        System.Console.ReadKey();
    }
}
// Output:
// 0
// 1
// 2
// 3
// 4
// ...
//

From the MSDN.